This invention relates to permanent magnet assemblies for use in medical applications and particularly to permanent magnet assemblies for use in Magnetic Resonance Imaging (MRI) and/or Magnetic Resonance Therapy (MRT) and/or interventional MRI (iMRI) which produce a predetermined volume of substantially uniform magnetic field extending in a first direction beyond the surface of the permanent magnet assemblies.
The principles of MRI are set forth in several patents such as U.S. Pat. No. 5,304,933, which is incorporated herein by reference. Interventional MRI (iMRI) which is also referred to as intraoperative MRI, is the performance of an interventional medical procedure on a patient in an MRI system. During the procedure, a surgical instrument is inserted into a patient in order to perform the procedure at a predetermined site in the body. The iMRI system is used in this case to monitor in quasi real-time the correct placement of the instrument and also to observe the nature and the extent of the effect of the intervention on the tissue.
In an MRI and/or iMRI system a strong uniform magnetic field is required in order to align an objects nuclear spins along the z-axis of a Cartesian coordinate system having mutually orthogonal x-y-z axes. The required strong uniform magnetic field, used for full body imaging, is normally in the order of 0.1 to 2 Tesla. The image quality and the accuracy of an MRI and/or iMRI system is dependent on the degree of uniformity of the strong uniform magnetic field. Uniformity is critical in MRI and/or iMRI applications because if the strong uniform magnetic field is not properly uniform within the volume of interest, the desired discrimination between different elements, due to the finely controlled magnetic field gradient, will be subject to misinterpretation. Typically, the uniformity required for the strong uniform magnetic field is within the order of 10 ppm within the volume of interest. It is essential for iMRI systems used in interventional procedures to be based on an open structure, so as to provide the physician easy access to the intervention site. Presently, most MRI systems employ a large magnet, which effectively surrounds the whole body of the patient, to produce the strong uniform magnetic field. Such magnets are usually large superconductor resistive or permanent magnets, each of which is expensive and heavy. Further, the access to the patient in these cases is obstructed.
Attempts have been made to provide open magnets for interventional procedures by employing two spaced-apart Helmholtz superconductive coil assemblies. They provide only limited space between the assemblies allowing for constricted access by only one person, such as a surgeon. Moreover, they are large, massive, immobile and expensive. See U.S. Pat. No. 5,410,287 (Laskaris et al.) and U.S. Pat. No. 5,428,292 (Dorri et al.).
U.S. Pat. No. 5,574,417 (Dorri et al.) discloses an open MRI magnet having two opposing superconducting coils and two annular shaped permanent magnet arrays. The radially separated inward and radially apart from the superconducting coil. Each of the permanent magnet arrays has a magnetic field direction generally opposite to the magnetic field direction of the corresponding superconducting coil. The permanent magnet arrays are stationary.
U.S. Pat. No. 5,696,476 (Havens et al.) discloses a passively shimmed open architecture magnetic resonance imaging magnet utilizing separated superconducting coil assemblies with open space between including shaped ferromagnetic bands in the bore of the magnets, the shaping including an axial notch and circumferential holes for attaching passive shims, the ferromagnetic bands are stationary.
U.S. Pat. No. 4,710,741 (McGinley et al.) discloses a coil arrangement for producing a homogenous magnetic field, comprising a single pair of identical annular coils disposed coaxially in spaced relationship and a pair of annular members of ferromagnetic material disposed coaxially with the coils. The annular members are stationary.
U.S. Pat. No. 5,463,364 (Muller) discloses an open magnetic apparatus having two opposing electromagnet coils and one or two pairs of opposing ferromagnetic rings. The rings of one pair of ferromagnetic rings are at least partially integrated into the electromagnet coils. The pairs of rings are stationary relative to the coils.
British patent GB 2219406A (Warner) discloses an electromagnet for providing a volume of homogenous magnetic field, comprising a pair of annular coils disposed coaxially in spaced relationship and ferromagnetic means, such as ferromagnetic annuli, disposed coaxially with the coils. The ferromagnetic means are stationary means.
U.S. Pat. No. 4,875,485 (Matsutani) discloses an apparently more compact configuration, based on a pair of spaced-apart superconductive Helmholtz coil assemblies, arranged for movement relative to a platform carrying the patient. The access to the patient remains restricted in this case as well, due to the additional space occupied by the cryostat. Additionally, the movement of the coils independently of one another is impractical, because the superconducting properties of the coils require extreme precision in positioning of the two poles, in the absence of which the magnetic system quenches.
In comparison to superconductive systems, permanent magnets are less expensive, generate only a minimal unwanted fringe field and are not involved with liquefied gas handling or vacuum requirements. Open access MRI systems based on permanent magnets have been disclosed by U.S. Pat. No. 4,829,252 (Kaufman) and U.S. Pat. No. 5,134,374 (Breneman). Both are using a pair of opposing magnetic flat circular poles, employed one above the other, with the patient lying down between the magnets. The poles are mounted on end plates, supported by connecting members, which provide return paths for the magnetic flux. These systems are massive and immobile and the access to the patient is encumbered by the supporting structure.
A pair of opposing permanent magnet assemblies for use in MRI, each made of concentric magnetic rings, composed of a set of magnetic polygonal blocks, is disclosed in U.S. Pat. No. 5,332,971 (Aubert). Aubert teaches that the opposing concentric rings within each of the pairs of permanent magnets are to be spaced apart from each other the same distance. The magnet is massive, weighing about 3 tons and is therefore not amenable to movement relative to a patient""s body.
In each of the above prior art magnets, used for providing the large uniform magnetic field for MRI and/or iMRI applications, the magnetic field is generated in a first stage as uniformly as possible. More uniformity is achieved subsequently by shimming.
Co-pending U.S. patent application Ser. No. 09/161,336, to Zuk et al., entitled xe2x80x9cMAGNETIC APPARATUS FOR MRIxe2x80x9d, filed Sep. 25, 1998, assigned to the assignee of the present invention, and the corresponding International Patent Application PCT/IL98/00463 published as International Publication No. WO 99/15914, the entire specification of which is incorporated herein by reference, disclose, inter alia, magnetic apparatus including an opposing pair of permanent magnetic assemblies defining an open region therebetween in which an organ or body part is positioned for imaging. The magnetic apparatus includes a plurality of gradient coils at least one of which is positioned outside of the open region.
Co-pending U.S. patent application Ser. No. 09/405,835 to Katznelson et al., filed Sep. 27, 1999, entitled xe2x80x9cYOKED PERMANENT MAGNET ASSEMBLIES FOR USE IN MEDICAL APPLICATIONSxe2x80x9d, assigned to the assignee of the present invention and incorporated herein by reference in its entirety discloses, inter alia, an MRI system having an open magnet including two spaced apart permanent magnet assemblies attached to an open ferromagnetic yoke.
U.S. Pat. No. 6,147,578 to Panfil et al. entitled xe2x80x9cA METHOD FOR DESIGNING OPEN MAGNETS AND OPEN MAGNETIC APPARATUS FOR USE IN MRI/MRT PROBESxe2x80x9d, assigned to the assignee of the present invention, and the corresponding International Patent Application PCT/IL99/00075, published as International Publication No. WO 99/40593, entitled xe2x80x9cA METHOD FOR DESIGNING OPEN MAGNETS AND OPEN MAGNETIC APPARATUS FOR USE IN MRI/MRT PROBESxe2x80x9d, both applications are incorporated herein by reference in their entirety, disclose, inter alia, an open iMRI magnet having an open ferromagnetic yoke and including two spaced apart permanent magnet assemblies having ferromagnetic collimators arranged to have an open region therebetween.
Co-pending U.S. patent application Ser. No. 09/274,671 to Katznelson et al., filed Mar. 24, 1999, entitled xe2x80x9cHYBRID MAGNETIC APPARATUS FOR USE IN MEDICAL APPLICATIONSxe2x80x9d, discloses, inter alia, a compact open iMRI magnet having adjustable permanent magnet assemblies and fixed electromagnet assemblies.
There is therefore provided, in accordance with a preferred embodiment of the present invention an open magnetic apparatus for producing a predetermined volume of substantially uniform magnetic field directed parallel to an axis of symmetry of the volume. The apparatus includes a first electromagnet assembly disposed at a first position along the axis. The first electromagnet assembly includes at least a first electromagnet coil. The at least first electromagnet coil is radially symmetric with respect to the axis. The apparatus also includes a second electromagnet assembly disposed at a second position spaced apart from the first position of the electromagnet assembly along the axis. The second electromagnet assembly includes at least a second electromagnet coil. The at least second electromagnet coil is radially symmetric with respect to the axis. The first electromagnet coil and the second electromagnet coil are substantially equidistant from the center of the volume. The first electromagnet assembly and the second electromagnet assembly are configured for generating a first magnetic field within the volume. The apparatus also includes a first permanent magnet assembly positioned at a third position disposed between the first position and the second position along the axis. The first permanent magnet assembly has an inner surface facing the volume and an outer surface facing the first electromagnet assembly. The first permanent magnet assembly includes at least two coaxial permanent magnets. The at least two permanent magnets are radially symmetric with respect to the axis. The apparatus also includes a second permanent magnet assembly opposed to the first permanent magnet assembly. The second permanent magnet assembly is positioned at a fourth position spaced apart from the third position along the axis. The second permanent magnet assembly has an inner surface facing the volume and an outer surface facing the second electromagnet assembly. The second permanent magnet assembly includes at least two coaxial permanent magnets the at least two permanent magnets of the second permanent magnet assembly are radially symmetric with respect to the axis. The inner surface of the first permanent magnet assembly and the inner surface of the second permanent magnet assembly define an open region therebetween. The volume is disposed within the open region. The third position of the first permanent magnet assembly and the fourth position of second permanent magnet assembly are substantially equidistant from the center of the volume along the axis. The first permanent magnet assembly and the second permanent magnet assembly are configured for generating a second magnetic field superimposed on the first magnetic field to provide the substantially uniform magnetic field within the volume.
Furthermore, in accordance with another preferred embodiment of the present invention, at least one electromagnet assembly of the first electromagnet assembly and the second electromagnet assembly is a movable electromagnetmagnet assembly.
Furthermore, in accordance with another preferred embodiment of the present invention, at least one electromagnet assembly of the first electromagnet assembly and the second electromagnet assembly is controllably movable relative to the volume.
There is further provided, in accordance with another preferred embodiment of the present invention, an open magnetic apparatus for producing a predetermined volume of substantially uniform magnetic field directed parallel to an axis of symmetry of the volume. The apparatus includes a first electromagnet assembly disposed at a first position along the axis. The first electromagnet assembly includes at least a first electromagnet coil. The at least first electromagnet coil is radially symmetric with respect to the axis. The apparatus further includes a second electromagnet assembly disposed at a second position spaced apart from the first position of the first electromagnet assembly along the axis. The second electromagnet assembly includes at least a second electromagnet coil. The at least second electromagnet coil is radially symmetric with respect to the axis. The at least first electromagnet coil and the at least second electromagnet coil are substantially equidistant from the center of the volume. The first electromagnet assembly and the second electromagnet assembly are configured for generating a first magnetic field within the volume. The apparatus further includes a first permanent magnet assembly positioned at a third position disposed between the first position and the second position along the axis. The first permanent magnet assembly has an inner surface facing the volume and an outer surface facing the first electromagnet assembly. The first permanent magnet assembly includes at least two coaxial permanent magnets. The at least two permanent magnets are radially symmetric with respect to the axis. The apparatus further includes a second permanent magnet assembly opposed to the first permanent magnet assembly. The second permanent magnet assembly is positioned at a fourth position spaced apart from the third position along the axis. The second permanent magnet assembly has an inner surface facing the volume and an outer surface facing the second electromagnet assembly. The second permanent magnet assembly includes at least two coaxial permanent magnets. The at least two permanent magnets of the second permanent magnet assembly are radially symmetric with respect to the axis. The inner surface of the first permanent magnet assembly and the inner surface of the second permanent magnet assembly define an open region therebetween. The volume is disposed within the open region. The third position of the first permanent magnet assembly and the fourth position of second permanent magnet assembly are substantially equidistant from the center of the volume along the axis. The first permanent magnet assembly and the second permanent magnet assembly are configured for generating a second magnetic field superimposed on the first magnetic field to provide the substantially uniform magnetic field within the volume. At least one of the first electromagnet assembly, the second electromagnet assembly, the first permanent magnet assembly and the second permanent magnet assembly is controllably movable relative to the volume.
Furthermore, in accordance with another preferred embodiment of the present invention, at least one electromagnet assembly of the first electromagnet assembly and the second electromagnet assembly is controllably movable relative to the volume.
Furthermore, in accordance with another preferred embodiment of the present invention, the hybrid magnetic apparatus further includes at least one moving mechanism configured for moving the at least one electromagnet assembly relative to the volume.
Furthermore, in accordance with another preferred embodiment of the present invention, the at least one moving mechanism is configured for moving the at least one electromagnet assembly towards or away from the volume in a direction parallel to the axis.
Furthermore, in accordance with another preferred embodiment of the present invention, the at least one moving mechanism is configured for tilting the at least one electromagnet assembly at an angle with respect to a plane orthogonal to the axis.
Furthermore, in accordance with another preferred embodiment of the present invention, the at least one moving mechanism is configured for moving the at least one electromagnet assembly towards or away from the volume in a direction parallel to the axis and for tilting the at least one electromagnet assembly at an angle with respect to a plane orthogonal to the axis.
Furthermore, in accordance with another preferred embodiment of the present invention, the at least one moving mechanism is a motorized moving mechanism.
Furthermore, in accordance with another preferred embodiment of the present invention, the motorized moving mechanism includes at least one motor for controllably moving the at least one electromagnet assembly.
Furthermore, in accordance with another preferred embodiment of the present invention, the at least one motor is a magnetic resonance imaging compatible motor.
Furthermore, in accordance with another preferred embodiment of the present invention, at least one permanent magnet assembly of the first permanent magnet assembly and the second permanent magnet assembly is movable relative to the volume.
Furthermore, in accordance with another preferred embodiment of the present invention, the hybrid magnetic apparatus further includes at least one moving mechanism configured for moving the at least one permanent magnet assembly relative to the volume.
Furthermore, in accordance with another preferred embodiment of the present invention, the at least one moving mechanism is configured for moving the at least one permanent magnet assembly towards or away from the volume in a direction parallel to the axis.
Furthermore, in accordance with another preferred embodiment of the present invention, the at least one moving mechanism is configured for tilting the at least one permanent magnet assembly at an angle with respect to a plane orthogonal to the axis.
Furthermore, in accordance with another preferred embodiment of the present invention, the at least one moving mechanism is configured for moving the at least one permanent magnet assembly towards or away from the volume in a direction parallel to the axis and for tilting the at least one permanent magnet assembly at an angle with respect to a plane orthogonal to the axis.
Furthermore, in accordance with another preferred embodiment of the present invention, the at least one moving mechanism is a motorized moving mechanism.
Furthermore, in accordance with another preferred embodiment of the present invention, the motorized moving mechanism includes at least one motor for controllably moving the at least one permanent magnet assembly.
Furthermore, in accordance with another preferred embodiment of the present invention, the at least one motor is a magnetic resonance imaging compatible motor.
Furthermore, in accordance with another preferred embodiment of the present invention, the at least first electromagnet coil and the at least second electromagnet coil are configured such that the radius of the at least first electromagnet coil is equal to the radius of the at least second electromagnet coil, and the distance between the at least first electromagnet coil and the at least second electromagnet coil is equal to the radius of the at least first electromagnet coil.
Furthermore, in accordance with another preferred embodiment of the present invention, the at least first electromagnet coil and the at least second electromagnet coil are configured such that the radius of the at least first electromagnet coil is equal to the radius of the at least second electromagnet coil, and the distance between the at least first electromagnet coil and the at least second electromagnet coil is greater than the radius of the at least first electromagnet coil.
Furthermore, in accordance with another preferred embodiment of the present invention, the at least first electromagnet coil and the at least second electromagnet coil are super-conducting electromagnet coils.
Furthermore, in accordance with another preferred embodiment of the present invention, the apparatus further includes at least a first cooling device for cooling the at least first electromagnet coil and a second cooling device for cooling the at least second electromagnet coil.
Furthermore, in accordance with another preferred embodiment of the present invention, the at least first cooling device and the at least second cooling device include a Dewar container or cryostat.
Furthermore, in accordance with another preferred embodiment of the present invention, the at least first cooling device and the at least second cooling device are selected from a low temperature gas operated cooling device, a low temperature liquefied gas based cooling device, an active cryo-cooling device, and a cooling device based on low pressure evaporative cooling of a liquefied gas.
Furthermore, in accordance with another preferred embodiment of the present invention, the super-conducting electromagnet coils include a low-temperature super-conducting material.
Furthermore, in accordance with another preferred embodiment of the present invention, the super-conducting electromagnet coils include a high-temperature super-conducting material.
Furthermore, in accordance with another preferred embodiment of the present invention, the at least two coaxial permanent magnets of the first permanent magnet assembly include a first permanent magnet having an outer diameter. The first permanent magnet has a first magnetization direction parallel to the axis. The at least two coaxial permanent magnets of the first permanent magnet assembly further include at least a second generally annular permanent magnet. The at least second permanent magnet has an inner diameter larger than the outer diameter of the first permanent magnet. The second permanent magnet has a second magnetization direction parallel to the axis. The at least two coaxial permanent magnets of the second permanent magnet assembly include a third permanent magnet having an outer diameter. The third permanent magnet has a magnetization direction equal to the first magnetization direction of the first permanent magnet. The at least two coaxial permanent magnets of the second permanent magnet assembly further include at least a fourth generally annular permanent magnet. The at least fourth permanent magnet has an inner diameter larger than the outer diameter of the third permanent magnet. The at least fourth permanent magnet has a magnetization direction equal to the second magnetization direction of the at least second permanent magnet.
Furthermore, in accordance with another preferred embodiment of the present invention, the first permanent magnet has a first surface facing the volume. The at least second permanent magnet has a second surface facing the volume. The third permanent magnet has a third surface facing the volume. The at least fourth permanent magnet has a first surface facing the volume. At least part of the second surface is offset from at least part of the first surface by a first distance along the axis, and at least part of the fourth surface is offset from at least part of the third surface by a second distance along the axis. The first distance is substantially equal to the second distance.
Furthermore, in accordance with another preferred embodiment of the present invention, the first permanent magnet has a first surface facing the volume. The at least second permanent magnet has a second surface facing the volume. The third permanent magnet has a third surface facing the volume. The at least fourth permanent magnet has a first surface facing the volume. At least part of the second surface is coplanar with at least part of the first surface, and at least part of the third surface is coplanar with at least part of the fourth surface.
Furthermore, in accordance with another preferred embodiment of the present invention, the first permanent magnet and the third permanent magnet are selected from a disc-like permanent magnet having a circular cross section in a plane perpendicular to the axis, a regular right polygonal prism-like permanent magnet having a regular polygonal cross-section in a plane perpendicular to the axis and having N sides, a ring-like annular permanent magnet and a annular right regular polygonal permanent magnet having N sides.
Furthermore, in accordance with another preferred embodiment of the present invention, N is equal to or larger than eight.
Furthermore, in accordance with another preferred embodiment of the present invention, the at least second and the at least fourth permanent magnets are selected from a ring-like annular permanent magnet and an annular right regular polygonal permanent magnet having N sides.
Furthermore, in accordance with another preferred embodiment of the present invention, N is equal to or larger than eight.
Furthermore, in accordance with another preferred embodiment of the present invention, the first permanent magnet and the third permanent magnet include a plurality of permanently magnetized segments attached to adjacent segments using a non-conductive adhesive, to reduce eddy currents.
Furthermore, in accordance with another preferred embodiment of the present invention, the segments are equi-angular segments.
Furthermore, in accordance with another preferred embodiment of the present invention, the at least second annular permanent magnet and the at least fourth annular permanent magnet include a plurality of permanently magnetized segments attached to adjacent segments using a non-conductive adhesive, to reduce eddy currents.
Furthermore, in accordance with another preferred embodiment of the present invention, the segments are equi-angular segments.
Furthermore, in accordance with another preferred embodiment of the present invention, the first permanent magnet assembly includes a first low magnetic permeability frame for supporting the first permanent magnet and the at least second permanent magnet, and the second permanent magnet assembly includes a second low magnetic permeability frame for supporting the third permanent magnet and the at least fourth permanent magnet.
Furthermore, in accordance with another preferred embodiment of the present invention, the first magnetization direction and the second magnetization direction are parallel to the axis and have the same polarity.
Furthermore, in accordance with another preferred embodiment of the present invention, the first magnetization direction and the second magnetization direction are parallel to the axis and have opposite polarities.
Furthermore, in accordance with another preferred embodiment of the present invention, the at least first electromagnet coil and the at least second electromagnet coil are selected from a circular coil and a regular polygonal shaped coil having N sides.
Furthermore, in accordance with another preferred embodiment of the present invention, N is equal to or larger than eight.
Furthermore, in accordance with another preferred embodiment of the present invention, the first permanent magnet assembly includes a first low magnetic permeability frame for supporting the at least two coaxial permanent magnets included in the first permanent magnet assembly, and the second permanent magnet assembly includes a second low magnetic permeability frame for supporting the at least two coaxial permanent magnets included in the second permanent magnet assembly.
Furthermore, in accordance with another preferred embodiment of the present invention, the first permanent magnet assembly includes a first adjustment mechanism configured for moving at least one of the at least two coaxial permanent magnets of the first permanent magnet assembly with respect to the volume.
Furthermore, in accordance with another preferred embodiment of the present invention, the second permanent magnet assembly includes a second adjustment mechanism configured for moving at least one of the at least two coaxial permanent magnets of the second permanent magnet assembly with respect to the volume.
Furthermore, in accordance with another preferred embodiment of the present invention, the apparatus includes a first gradient coil assembly disposed between the first permanent magnet assembly and the first electromagnet assembly, and a second gradient coil assembly disposed between the second permanent magnet assembly and the second electromagnet assembly.
Furthermore, in accordance with another preferred embodiment of the present invention, each of the first gradient coil assembly and the second gradient coil assembly are non-movably attached within the hybrid magnetic apparatus.
Furthermore, in accordance with another preferred embodiment of the present invention, each of the first gradient coil assembly and the second gradient coil assembly includes one or more gradient coils selected from a x-gradient coil, a y-gradient coil, a z-gradient coil, and any combination thereof.
Furthermore, in accordance with another preferred embodiment of the present invention, each of the first gradient coil assembly and the second gradient coil assembly is a multi-layer printed circuit assembly. At least one of the x-gradient coil, y-gradient coil and z-gradient coil of each of the first gradient coil assembly and the second gradient coil assembly is a substantially planar printed circuit coil.
Furthermore, in accordance with another preferred embodiment of the present invention, the first gradient coil assembly is attached to the first electromagnet assembly.
Furthermore, in accordance with another preferred embodiment of the present invention, the first gradient coil assembly is attached to the first permanent magnet assembly.
Furthermore, in accordance with another preferred embodiment of the present invention, the first gradient coil assembly is attached to a support member. The support member is suitably attached to the hybrid magnetic apparatus.
Furthermore, in accordance with another preferred embodiment of the present invention, the second gradient coil assembly is attached to the second electromagnet assembly.
Furthermore, in accordance with another preferred embodiment of the present invention, the second gradient coil assembly is attached to the second permanent magnet assembly.
Furthermore, in accordance with another preferred embodiment of the present invention, the second gradient coil assembly is attached to a support member. The support member is suitably attached to the hybrid magnetic apparatus.
Furthermore, in accordance with another preferred embodiment of the present invention, the apparatus includes one or more support members attached to the first electromagnet assembly, the second electromagnet assembly, the first permanent magnet assembly, and the second permanent magnet assembly, for supporting the first electromagnet assembly, the second electromagnet assembly, the first permanent magnet assembly, and the second permanent magnet assembly within the apparatus.
Furthermore, in accordance with another preferred embodiment of the present invention, the apparatus further includes an open high magnetic permeability yoke configured for closing the magnetic field lines of the hybrid magnetic apparatus.
Furthermore, in accordance with another preferred embodiment of the present invention, the open high magnetic permeability yoke includes at least one material selected from a ferromagnetic material, a high magnetic permeability material, soft iron, a nickel-iron containing alloy, a silicon-iron containing alloy, low carbon iron, and combinations thereof.
Furthermore, in accordance with another preferred embodiment of the present invention, the open high magnetic permeability yoke is selected from a generally c-shaped yoke, a generally U-shaped yoke, and a generally Y-shaped yoke.
There is further provided, in accordance with another preferred embodiment of the present invention, a method for constructing an open magnetic apparatus for producing a predetermined volume of substantially uniform magnetic field directed parallel to an axis of symmetry of the volume. The method includes the step of providing a first electromagnet assembly disposed at a first position along the axis. The first electromagnet assembly includes at least a first electromagnet coil. The at least first electromagnet coil is radially symmetric with respect to the axis. The method also includes the step of providing a second electromagnet assembly disposed at a second position spaced apart from the first position of the electromagnet assembly along the axis. The second electromagnet assembly includes at least a second electromagnet coil. The at least second electromagnet coil is radially symmetric with respect to the axis. The at least first electromagnet coil and the at least second electromagnet coil are substantially equidistant from the center of the volume. The first electromagnet assembly and the second electromagnet assembly are configured for generating a first magnetic field within the volume. The method also includes the step of providing a first permanent magnet assembly positioned at a third position disposed between the first position and the second position along the axis. The first permanent magnet assembly has an inner surface facing the volume and an outer surface facing the first electromagnet assembly. The first permanent magnet assembly includes at least two coaxial permanent magnets. The at least two permanent magnets are radially symmetric with respect to the axis. The method also includes the step of providing a second permanent magnet assembly opposed to the first permanent magnet assembly. The second permanent magnet assembly is positioned at a fourth position spaced apart from the third position along the axis. The second permanent magnet assembly has an inner surface facing the volume and an outer surface facing the second electromagnet assembly. The second permanent magnet assembly includes at least two coaxial permanent magnets. The at least two permanent magnets of the second permanent magnet assembly are radially symmetric with respect to the axis. The method also includes the step of positioning the first permanent magnet assembly and the second permanent magnet assembly such that the inner surface of the first permanent magnet assembly and the inner surface of the second permanent magnet assembly define an open region therebetween. The volume is disposed within the open region. The third position of the first permanent magnet assembly and the fourth position of second permanent magnet assembly are substantially equidistant from the center of the volume along the axis. The first permanent magnet assembly and the second permanent magnet assembly generate a second magnetic field superimposed on the first magnetic field to provide the substantially uniform magnetic field within the volume.
Furthermore, in accordance with another preferred embodiment of the present invention, the method further includes the step of providing an open high magnetic permeability yoke configured for closing the magnetic field lines of the hybrid magnetic apparatus.
There is further provided, in accordance with another preferred embodiment of the present invention, a method for tuning an open magnetic apparatus for producing a predetermined volume of substantially uniform magnetic field directed parallel to an axis of symmetry of the volume. The method includes the step of providing a first electromagnet assembly disposed at a first position along the axis, the first electromagnet assembly includes at least a first electromagnet coil, the at least first electromagnet coil is radially symmetric with respect to the axis. The method also includes the step of providing a second electromagnet assembly disposed at a second position spaced apart from the first position of the electromagnet assembly along the axis, the second electromagnet assembly includes at least a second electromagnet coil, the at least second electromagnet coil is radially symmetric with respect to the axis, the at least first electromagnet coil and the at least second electromagnet coil are substantially equidistant from the center of the volume, the first electromagnet assembly and the second electromagnet assembly are configured for generating a first magnetic field within the volume. The method also includes the step of providing a first permanent magnet assembly positioned at a third position disposed between the first position and the second position along the axis, the first permanent magnet assembly has an inner surface facing the volume and an outer surface facing the first electromagnet assembly, the first permanent magnet assembly includes at least two coaxial permanent magnets, the at least two permanent magnets are radially symmetric with respect to the axis. The method also includes the step of providing a second permanent magnet assembly opposed to the first permanent magnet assembly, the second permanent magnet assembly is positioned at a fourth position spaced apart from the third position along the axis, the second permanent magnet assembly has an inner surface facing the volume and an outer surface facing the second electromagnet assembly, the second permanent magnet assembly includes at least two coaxial permanent magnets, the at least two permanent magnets of the second permanent magnet assembly are radially symmetric with respect to the axis. The method also includes the step of positioning the first permanent magnet assembly and the second permanent magnet assembly such that the inner surface of the first permanent magnet assembly and the inner surface of the second permanent magnet assembly define an open region therebetween. The volume is disposed within the open region. The third position of the first permanent magnet assembly and the fourth position of second permanent magnet assembly are substantially equidistant from the center of the volume along the axis. The first permanent magnet assembly and the second permanent magnet assembly generate a second magnetic field superimposed on the first magnetic field to provide the substantially uniform magnetic field within the volume. The method also includes the step of controllably moving at least one of the first electromagnet assembly, the second electromagnet assembly, the first permanent magnet assembly, and the second permanent magnet assembly relative to the volume for improving the homogeneity of the magnetic field within the volume.
Furthermore, in accordance with another preferred embodiment of the present invention, the method further includes the step of providing an open high magnetic permeability yoke configured for closing the magnetic field lines of the hybrid magnetic apparatus, prior to the step of controllably moving.
There is further provided, in accordance with another preferred embodiment of the present invention, a method for operating an open magnetic apparatus for producing a predetermined volume of substantially uniform magnetic field directed parallel to an axis of symmetry of the volume. The method includes the step of providing a first electromagnet assembly disposed at a first position along the axis. The first electromagnet assembly includes at least a first electromagnet coil, the at least first electromagnet coil is radially symmetric with respect to the axis. The method also includes the step of providing a second electromagnet assembly disposed at a second position spaced apart from the first position of the electromagnet assembly along the axis. The second electromagnet assembly includes at least a second electromagnet coil. The at least second electromagnet coil is radially symmetric with respect to the axis. The at least first electromagnet coil and the at least second electromagnet coil are substantially equidistant from the center of the volume. The first electromagnet assembly and the second electromagnet assembly are configured for generating a first magnetic field within the volume. The method also includes the step of providing a first permanent magnet assembly positioned at a third position disposed between the first position and the second position along the axis. The first permanent magnet assembly has an inner surface facing the volume and an outer surface facing the first electromagnet assembly. The first permanent magnet assembly includes at least two coaxial permanent magnets. The at least two permanent magnets are radially symmetric with respect to the axis. The method also includes the step of providing a second permanent magnet assembly opposed to the first permanent magnet assembly. The second permanent magnet assembly is positioned at a fourth position spaced apart from the third position along the axis. The second permanent magnet assembly has an inner surface facing the volume and an outer surface facing the second electromagnet assembly. The second permanent magnet assembly includes at least two coaxial permanent magnets. The at least two permanent magnets of the second permanent magnet assembly are radially symmetric with respect to the axis. The method also includes the step of positioning the first permanent magnet assembly and the second permanent magnet assembly such that the inner surface of the first permanent magnet assembly and the inner surface of the second permanent magnet assembly define an open region therebetween. The volume is disposed within the open region. The third position of the first permanent magnet assembly and the fourth position of second permanent magnet assembly are substantially equidistant from the center of the volume along the axis. The first permanent magnet assembly and the second permanent magnet assembly generate a second magnetic field superimposed on the first magnetic field to provide the substantially uniform magnetic field within the volume. The method also includes the step of electrically energizing the first electromagnet assembly and the second electromagnet assembly to provide a second magnetic field within the volume. The second magnetic field is superimposed on the first permanent magnetic field to provide the substantially uniform magnetic field within the volume.
Furthermore, in accordance with another preferred embodiment of the present invention, the method further includes the step of providing an open high magnetic permeability yoke configured for closing the magnetic field lines of the hybrid magnetic apparatus.
Furthermore, in accordance with another preferred embodiment of the present invention, the method further includes the step of controllably moving at least one of the first electromagnet assembly, the second electromagnet assembly, the first permanent magnet assembly, and the second permanent magnet assembly relative to the high magnetic permeability yoke for improving the homogeneity of the magnetic field within the volume.
Finally, in accordance with another preferred embodiment of the present invention, the open high magnetic permeability yoke includes at least one material selected from a ferromagnetic material, a high magnetic permeability material, soft iron, a nickel-iron containing alloy, a silicon-iron containing alloy, low carbon iron, and combinations thereof.